I would just start speaking Spanish to the children. Children learn amazingly quickly. In our home, I speak only French to DS and have since he was born. Much to my dismay, my husband - who is fluent in Spanish although not a native speaker - did not start speaking Spanish until a few months before DS took his first trip with us to Central America at age three. Fast forward three years and DS is now completely comfortable speaking Spanish, speaks exclusively in Spanish to DH, and gets upset if his father speaks to him in anything but Spanish.
So DH and I do OPOL (one parent one language) with DS but speak English to each other. It works out very well. We spend part of the summer in a Spanish speaking country, which helps reinforce the Spanish. We also read mostly in French and Spanish, and what little video watching takes place is exclusively in French or Spanish.
Good luck! It might seem a little awkward at first switching to another language (this is what DH told me in the beginning), but soon it will be natural and, of course, the benefits for the children are enormous.